Methylene groups ketones, oxidation, allylic

Oxidations by pyridinium chlorochromate resemble those by dipyridine chromium(VI) oxide, both in scope and the mild conditions required. At room temperature, primary alcohols give aldehydes [604, 605], secondary alcohols afford ketones [605], allylic and benzylic methylene groups are oxidized to carbonyl groups [606, 6d7], enol ethers are converted into esters [608] or lactones [609], trimethylsilyl ethers of diphenols are transformed into quinones [610], and alkylboranes are converted into aldehydes (yll]. 

For the oxidation of allylic methylene groups (C=C-CH2 C=C-C=0), see Section 170 (Ketones from Alkyls and Methylenes). 

Freshly prepared Mn02 is a useful reagent in organic chemistry and has been used in a large variety of oxidative transformations.311 These reactions involve the allylic oxidation of alkene to a,/3-unsaturated carbonyl compounds, the transformation of methylarenes to benzaldehyde and benzoic acid derivatives, the oxidation of secondary methylene groups to ketones, and the oxidation of alcohols to carbonyl compounds.311 The yields are generally fair to good. 

Selenium dioxide (SeOi) oxidation Selenium dioxide is an excellent oxidizing agent for the oxidation of allylic and benzylic C-H fragments to allylic or benzylic alcohol. It also oxidizes the aldehydes and ketones to 1,2-dicarbonyl compounds (i.e. oxidation of active methylene groups to carbonyl groups). 

Air, the cheapest oxidant, is used only rarely without irradiation and without catalysts. Examples of oxidations by air alone are the conversion of aldehydes into carboxylic acids (autoxidation) and the oxidation of acyl-oins to a-diketones. Usually, exposure to light, irradiation with ultraviolet light, or catalysts are needed. Under such circumstances, dehydrogenative coupling in benzylic positions takes place at very mild conditions [7]. In the presence of catalysts, terminal acetylenes are coupled to give diacetylenes [2], and anthracene is oxidized to anthraquinone [3]. Alcohols are converted into aldehydes or ketones with limited amounts of air [4, 5, 6, 7], Air oxidizes esters to keto esters [3], thiols to disulfides [9], and sulfoxides to sulfones [10. In the presence of mercuric bromide and under irradiation, methylene groups in allylic and benzylic positions are oxidized to carbonyls [11]. 

The presence of an exocyclic methylene group in ignavine is indicated by the IR spectrnm (1645 and 892 cm i) and by formation of formaldehyde on ozonization. That the exocyclic methylene is involved in a secondary allylic alcohol system as in atisine was demonstrated by catalytic isomerization to a methyl ketone (1692 cm i) and by oxidation to a conjugated enone (1615, 1687 cm i) (72). 

The intermediate ketone (7.49) possesses the same spirane system as the target and it remains only to remove one oxygen atom, introduce one other, saturate the double bond and add an isopropylidene group. The two functions to be added must, of course be added at the correct sites and it can be seen that the existing functionalisation of (7.49) will allow this to be done selectively. The methylene group to be oxidised is allylic and so the oxidation will be relatively easy. The isopropylidene group must be added adjacent to the ketone of (7.49) and selectively on one side of it. Steric effects allow the required selectivity. The complete synthesis is outlined in Figure 7.12. 

Most allylic oxidations of methylene to carbonyl groups are carried out by chromic oxide [552, 555] or its complexes with pyridine [593, 606] and performed on steroidal esters [552, 593, 606] and ketones [61, 552] (equation 130). 

---